Tunable MEMS capacitor

ABSTRACT

A MEMS tunable capacitor comprises first and second opposing capacitor electrodes, wherein the second capacitor electrode is movable by a MEMS switch to vary the capacitor dielectric spacing, and thereby tune the capacitance. A tunable dielectric material and a non-tunable dielectric material are in series between the first and second electrodes. The tunable dielectric material occupies a dimension g d  of the electrode spacing, and the non-tunable dielectric material occupies a dimension g of the electrode spacing. A third electrode faces the movable second electrode for electrically controlling tunable dielectric material. A controller is adapted to vary the capacitor dielectric spacing for a first continuous range of adjustment of the capacitance of the MEMS capacitor, and to tune the dielectric material for a second continuous range of adjustment of the capacitance of the MEMS capacitor, thereby to provide a continuous analogue range of adjustment including the first and second ranges. This arrangement provides independent control of the MEMS function and the dielectric tuning function, and enables a continuous adjustability.

This invention relates to tunable capacitors, in particular based oncapacitive MEMS structures.

Tunable capacitors can be used in a variety of circuits, such as tunablefilters, tunable phase shifters and tunable antennas. One application ofgrowing interest is in RF and microwave communications systems, forexample for use in low cost reconfigurable/tunable antennas.

Two of the most promising technologies for making tunable or switchableRF capacitors are RF MEMS switches and relays which provide mechanicalalteration to the capacitor spacing, and capacitors with electricallytunable dielectrics.

RF MEMS switches have the advantage of a larger capacitance switchingratio, and tunable dielectrics have the advantage of better continuouscapacitance tunability.

It has been proposed to combine these effects, by providing control ofthe dielectric spacing using a MEMS switch, in combination with aferroelectric tunable dielectric, such as barium strontium titanate(BST). The combination of discrete control provided by a MEMS switch andanalogue electrical control of the dielectric properties can enablecontinuous tunability of the capacitor. This approach is described inthe article “A High Performance Tunable RF MEMS Switch Using BariumStrontium Titanate (BST) Dielectrics for Reconfigurable Antennas andPhase Arrays” by Guong Wang et al., IEEE Antennas and WirelessPropagation Letters Vol. 4, 2005 pp 217-220.

FIG. 1 is used to explain how the electrically tunable dielectric andMEMS controlled dielectric spacing can be combined.

A tunable dielectric, ferroelectric or piezoelectric material can beused, such as Ba_(1-x)Sr_(x)TiO₃ or PZT as a dielectric layer 14. Bycombining a MEMS capacitor with a tunable dielectric, the advantages ofthe large capacitance switching range or RF MEMS switches are added tothe advantages of the continuous tuning capability of tunabledielectrics. Moreover use is made of the beneficial high dielectricconstant of ferroelectrics, which can be 10-200 times higher than thatof conventional dielectrics like Silicon Nitride. This dramaticallyreduces device size and increases continuous tuning range.

The device comprises opposite capacitor plates 10 (e 1)) and 12 (e 2).The gap g is controlled by the MEMS switch represented by the spring k,based on the voltage applied to the plate 12. A dc voltage Vdc_switch isused to provide this MEMS switching function, from a dc voltage source18. An rf ac voltage source 16 represents the rf signal that is flowingthrough the MEMS device during operation. The tunable dielectric has atunable dielectric value ∈_(d), whereas the remaining dielectric spacingis air or vacuum, with dielectric value ∈₀. The tunable dielectric iscontrolled by the voltage Vdc_tune, so that the single voltage appliedto the electrode 12 controls the MEMS switching and dielectric tuning.The capacitor C and resistor R are optional decoupling components.

FIG. 2 shows the tunable dielectric characteristic curve. The maximumdielectric constant occurs if the voltage across the tunable dielectric(Ubias) is zero volts. In the configuration of FIG. 1 this maximum cannever be reached because the MEMS switch will release before 0V.

According to the invention, there is provided a MEMS tunable capacitorcomprising:

first and second opposing capacitor electrodes, wherein the secondcapacitor electrode is movable by a MEMS switch to vary the capacitordielectric spacing, and thereby tune the capacitance;

a tunable dielectric material and a non-tunable dielectric material inseries between the first and second electrodes, wherein the tunabledielectric material occupies a first dimension of the electrode spacing,and the non-tunable dielectric material occupies a second dimension ofthe electrode spacing;

a third electrode facing the movable second electrode for electricallycontrolling tunable dielectric material; and

a controller,

wherein the controller is adapted to vary the capacitor dielectricspacing for a first continuous range of adjustment of the capacitance ofthe MEMS capacitor, and to tune the dielectric material for a secondcontinuous range of adjustment of the capacitance of the MEMS capacitor,thereby to provide a continuous analogue range of adjustment includingthe first and second ranges.

The invention thus provides a relay type arrangement for the capacitorunder the control of the third electrode, with independent control ofthe dielectric properties. The device has continuous adjustability ofcapacitance.

The non-tunable dielectric dimension is preferably less than one thirdof the total effective actuation electrode spacing when the movableelectrode is at the position corresponding to maximum electrode spacing.

This gap design enables a continuous adjustability of capacitance bypreventing pull-in of the movable electrode during capacitor tuning.

The tunable dielectric material is preferably a solid and thenon-tunable dielectric material is a gas. Thus, the movement of thesecond electrode displaces the gas dielectric (e.g. air or a vacuum).

The movable electrode can be at a position corresponding to minimumelectrode spacing when the electrode spacing has dimension g_(d), namelywith only the tunable dielectric sandwiched between the first and secondelectrodes.

The tunable dielectric material preferably comprises a ferroelectricmaterial, such as BST.

The ac rf voltage source can be used to control the tunable dielectricmaterial, by means of a dc component added to the rf signal in thecircuit in which the capacitor is used. A dc voltage source can be usedfor controlling the MEMS switching function.

The first and third electrodes can be provided on a static substrate,and the second electrode comprises a cantilever structure suspended overthe static substrate. In one arrangement, only the first electrode iscovered by the tunable dielectric material, or in another arrangementthe first and third electrodes are covered by the tunable dielectricmaterial. In the latter case, a fourth electrode can be provided overthe tunable dielectric only above the first electrode.

The first and second electrodes can be flat, but the movable secondelectrode can instead be shaped such that a different gap is providedbetween the first and second electrodes and between the second and thirdelectrodes. The MEMS switching function and dielectric tuning functioncan thus each be optimised.

The invention also provides a tunable antenna comprising an antennadevice, a first tunable circuit including a capacitor of the inventionfor a transmission channel and a second tunable circuit including acapacitor of the invention for a reception channel.

The capacitors of the invention can also be used in a tunable capacitornetwork comprising a plurality of tunable capacitors or a plurality ofstatic capacitors and at least one tunable capacitor in parallel.

Examples of the invention will now be described in detail with referenceto the accompanying drawings, in which:

FIG. 1 is used to explain how the electrically tunable dielectric andMEMS controlled dielectric spacing can be combined;

FIG. 2 shows the tunable dielectric characteristic curve for thearrangement of FIG. 1;

FIG. 3 shows a first possible arrangement with independent control ofthe MEMS switch and dielectric;

FIG. 4 shows an improvement to the design of FIG. 3 in accordance withthe invention;

FIG. 5 is a sketch of the capacitance-voltage (C-V) characteristic ofthe configuration of FIG. 1;

FIG. 6 shows the capacitance characteristics for the configuration inFIG. 3;

FIG. 7 shows the improved capacitance characteristics provided by thearrangement of FIG. 4;

FIG. 8 shows a first alternative implementation in accordance with theinvention;

FIG. 9 shows a second alternative implementation in accordance with theinvention;

FIG. 10 shows a third alternative implementation in accordance with theinvention;

FIG. 11 shows a fourth alternative implementation in accordance with theinvention;

FIG. 12 is used to show a more general relationship for the gaprelationships in accordance with the invention;

FIG. 13 shows capacitive MEMS switches of the invention used in atunable antenna;

FIG. 14 shows tunable capacitors of the invention in parallel to extendthe tunable range;

FIG. 15 shows the continuous tuning of parallel combinations of switchesas shown in FIG. 14;

FIG. 16 shows a first way of implementing the movable electrode; and

FIG. 17 shows a second way of implementing the movable electrode.

The invention provides a MEMS relay device, with a tunable dielectric,for example a ferroelectric or other tunable dielectric material such asBa_(1-x)Sr_(x)TiO₃ or PZT.

In the figures, the structure of the device is shown only schematically.In particular, the way the top electrode is formed and the way it moveshas not been shown. In one known example. the top electrode can beformed as a suspended beam which is connected to the lower substrate atone lateral end. The detailed implementation will be routine to thoseskilled in the art of MEMS devices.

A first possible arrangement is shown in FIG. 3. The implementation of acapacitive relay is achieved by electrically separating the electrode 20(e 3) (which receives the high voltage which generates the electrostaticforce that moves the electrode of the MEMS capacitor) from theelectrodes 10,12 which form the RF capacitor (e1 and e2). When the MEMSswitch is closed under the control of Vdc_switch, the dielectricconstant ∈_(d) can be tuned by applying a DC voltage Vdc_tune betweenelectrodes e1 and e2. This results in a continuously tunable capacitor.The switch can remain closed regardless of the tuning voltage, becausethere is independent control of the switch (i.e. the dielectric spacing)and of the dielectric properties. The separated relay electrode 20actuates the MEMS switch and thus controls movement of the electrode 12(e 2).

For example, the voltage vdc_tune can range from 0-5 V and vdc_switchcan range from 0-50 V.

FIG. 4 shows an improvement to this basic design and which providesincreased continuous tuning range while requiring only one tuningvoltage control.

The arrangement comprises the same components as in FIG. 3, but thetunable dielectric is chosen to occupy most of the space between thecapacitor electrodes used for dielectric tuning, in particular so thatthe remaining gap g is less than one third of the total gap. Theresistor R2 and capacitor C are again optional.

In the example of FIG. 4, the tunable dielectric 14 is provided onlyover the electrode 10.

Because there is a tunable dielectric and a MEMS switch in the device,there are two relevant tuning ranges. Firstly there is the MEMScapacitance switching or tuning ratio α_(MEMS)=C_(on)/C_(off) if thedevice is purely used as a MEMS device. Secondly there is the tuningratio of the tunable dielectric capacitorα_(TD)=C_(max)/C_(min)=∈_(d)(E_(max))/∈_(d)(0).

In the switch implementation of FIG. 1, the useful tuning range of thedevice in the closed state is less than α_(TD)=C_(max)/C_(min) becausethe voltage across the dielectric cannot be brought to zero withoutrelease of the MEMS device. The useful tuning range of the basic switchof FIG. 1 is indicated in FIG. 5.

FIG. 5 is a sketch of the capacitance-voltage (C-V) characteristic ofthe configuration of FIG. 1. After closing the switch (i.e. with highvalue of V above the level shown as 52), the capacitance of the devicecan be tuned continuously over a range which is less than the maximumtunable range of the tunable dielectric. When V drops below a certainvalue (shown as 54), the MEMS device switches, so that the maximumcapacitance cannot be reached (region 50). The value V corresponds toVdc_switch+Vdc_tune in FIG. 1 and C is the RF capacitance in FIG. 1between electrodes e1 and e2.

FIG. 6 shows the capacitance versus Vdc_tune and versus Vdc_switch forthe configuration in FIG. 3. A larger continuous tuning range isachievable. The left plot in FIG. 6 shows the capacitance characteristicwhen the MEMS switch is controlled. The MEMS switch closes atVdc_switch=Vpi and there is thus the step change in capacitance. Thedevice has hysteresis so that it switches back to open at a lowervoltage. The capacitance can also be tuned by more than 30% in the openstate (Vdc_switch<Vpi). The right plot in FIG. 6 shows the capacitancecharacteristic when the dielectric is controlled with the switch closed(Vdc_switch>Vpi). There is no forbidden region such as 50 in FIG. 5.

In the implementation of the FIG. 4, the condition in equation (1) issatisfied, as mentioned above:g<(g+g _(d))/3  Equation (1)

A MEMS capacitor will show pull in when it has traveled one third of thegap used for the MEMS switch actuation (i.e. the gap between electrodes12 and 20). The design of the invention takes this into account, in sucha way that the top plate will touch the dielectric before pull-inoccurs. Therefore, in the geometry of FIG. 4, full continuous tuning canbe achieved. In this way, the electrode 12 is mechanically blocked frommoving beyond the point at which pull-in occurs. This can be done bymaking the dielectric 14 thick as shown in FIG. 4, but it can also beachieved by using separate blocking stubs, as will be apparent fromfurther examples below. These approaches can also be combined, withstubs preventing bending of the electrode 12, and the contact with thedielectric 14 in the blocking position giving the highest capacitancevalue.

The invention thus provides a MEMS switch and a tunable dielectriccapacitor which are combined in such a way that both show continuoustuning. Instead of a combination of digital and analog tuning, completeanalog tuning becomes possible.

FIG. 7 shows the improved performance provided by the invention andshows plots corresponding to FIG. 6. In this arrangement, continuoustuning over the full capacitance range is achieved. Furthermore, theslope of the Vdc_tune graph is less which allows more accurate tuning.Moreover, when large RF powers are present it is more favorable to useVdc_tune than Vdc_switch to tune the capacitance value to reducenon-linearities.

Thus, for the capacitive MEMS relay of FIG. 4, two methods of tuning thecapacitance are possible. First continuous tuning is possible usingVdc_switch. This can tune over a large capacitance range, but has twodisadvantages:

-   -   A large slope in the C-V curves makes accurate tuning difficult.    -   High RF voltage between electrodes 10 and 12 will result in an        additional force, which changes the C-Vdc_switch curve and makes        accurate control of the capacitance difficult. Moreover, this        generates non-linearities.

The tunable dielectric does not have these problems. Thus, to cover thefull-range of tunability, Vdc_switch should be used to tune the small Cvalues, and for capacitance values larger than a minimum value, Vdc_tuneshould be used with Vdc_switch>Vpi (the right graph of FIG. 7).

In combination with the large dielectric constants of the tunabledielectrics this can allow continuous tuning of the capacitance by afactor of 500.

A controller is used to drive the capacitor, and thereby set the desiredcapacitance. In accordance with the invention, the controller is adaptedto vary the capacitor dielectric spacing for a first continuous range ofadjustment of the capacitance of the MEMS capacitor, and to tune thedielectric material for a second continuous range of adjustment of thecapacitance of the MEMS capacitor.

The first range is controlled by Vdc_switch, until the MEMS switch isclosed, and the second range is controlled by Vdc_tune.

The two ranges are combined to provide a full continuous adjustablerange, for example with a ratio of more than 100, 200, 300 or even morethan 500.

FIG. 8 shows an alternative implementation in which the tunabledielectric covers all of the fixed plate electrodes. A differentdielectric can be used on the electrodes 20 compared to the electrode10, because the required properties of the dielectric on electrode 20are different.

FIG. 9 shows a modification in which both capacitor plates are providedwith separate electrodes for the dielectric tuning and for the MEMSswitching. Thus, each capacitor plate comprises a MEMS switch electrode20 and 92, and these face each other, and each capacitor plate has a setof dielectric tuning electrodes 10 and 94, which again face each other.As schematically shown, the movable electrode has an insulatingsubstrate 90, so that independent voltages can be provided to the twosets of electrodes 92,94 on the movable plate.

FIG. 10 shows another implementation in which a further electrode 100 (e4) is provided on top of the tunable dielectric layer, so that when theMEMS switch is closed, a galvanic contact between the movable electrode12 (e 2) and the additional electrode 100 in the closed state willreduce the effect of electrode roughness on the capacitance density.

FIG. 11 shows a further alternative implementation in which the movableplate is not flat, but has a raised portion 110 which acts to reduce thecapacitor gap in the region having the tunable dielectric. This againprevents collapse by reducing the air gap.

The shaped profile generates and sustains gap variations and springconstant variations. Reference is made to WO 2006/046193 andWO2006/046192 for further discussion in connection with a shaped movableelectrode in a MEMS device.

FIG. 12 is used to show a more general relationship for the gaprelationships in accordance with the invention.

The movable plate is shown as having different thickness for differentparts of the structure. In addition, the dielectric thickness isdifferent over the MEMS switching electrodes and the dielectric tuningelectrode.

The relationship for the permitted gap g which is closed by the MEMSswitch is given by:g<(g2+gd2/∈d2)/3  Equation (2)

This corresponds to equation (1), in that the effective actuation gapdimension is reduced to one third. The effective gap is taken to beg2+gd2/∈d2. Note that if there is no dielectric layer in the actuationpath, then ∈d2=1 and Equation (2) simplifies to Equation (1), asg2+gd2=g+gd.

As mentioned above, one of the main applications of MEMS capacitors andtunable dielectrics is in tunable filters in the front-end of RFcommunications devices, such as mobile phones. Because the MEMScapacitor is switching the capacitance over a large ratio, it caneffectively act as a switch. The tunable dielectric can be used forfine-tuning the filter frequency to the desired value. This is forexample useful for implementing tunable filters and transmit/receiveswitches in mobile front-end of a mobile phone as shown in FIG. 13.

FIG. 13 shows two capacitive MEMS switches 130,132 with tunabledielectrics (indicated by a switch and a tunable capacitor) to providetunable filters in transmit (Tx) and receive (Rx) channels in the pathof an antenna 134. The switches provide isolation between Rx and Txchannels.

By putting several switch and tunable dielectrics in parallel, thecontinuous tuning range can be extended significantly. It can be assumedthat the switching ratio of the MEMS switch is much larger than thetuning ratio of the tunable dielectric (this is usually the case). Ifthe tunable dielectric has a continuous tuning ratio of 2, this tuningratio can be increased by putting several of the proposed devices inparallel as is shown in FIGS. 14 and 15.

FIG. 14 shows three devices in parallel with capacitances C0, 2C0 and6C0.

A continuous tuning of the tunable dielectric of a factor 2 is assumed.Capacitor values have been chosen to maximize the continuous tuningrange from C0-18C0. The continuous tuning range is thus a factor 18.

The off capacitance of the MEMS switch is assumed to be negligiblecompared to the smallest parallel capacitance C0. The circuit in FIG. 14can be a single device or it might be made using separate MEMS switchesand tunable capacitors (although this would require more space).

A tuning range with a factor 17 could also be made using one device witha tunable dielectric (C0-2C0) in parallel with 4 switchable MEMScapacitors with capacitance values C0, 2C0, 4C0 and 8C0. This wouldrequire 5 devices instead of 3, but would only require one tunabledielectric device.

FIG. 15 shows the continuous tuning of parallel combinations of switchesas shown in FIG. 14. The continuous tuning range extends from C0 to 18C0. Different curves correspond to different settings of the MEMSswitches using the MEMS actuation voltage.

The invention thus enables the advantage of the large switching andtuning ratio of capacitive MEMS relays to be combined with the advantageof large continuous tuning at high power levels provided by tunabledielectric materials. A larger continuous tuning range can thus beobtained with better power handling and linearity. The PZT or BST high-kdielectrics also allow larger capacitance density in the closed stateand thus device size reduction.

This size reduction also reduces parasitic resistances and inductances.

A switching and tuning function can be combined and controlled usingseparated voltages.

The structure of the movable beam has not been described in detailabove. It can be favorable to have more than one spring/suspensionarrangement.

FIG. 16 shows an arrangement in top view, with 4 springs, fixed at thepoints 160. FIG. 17 shows an arrangement in top view, with 8 springs.

Various modifications will be apparent to those skilled in the art.

The invention claimed is:
 1. A MEMS tunable capacitor comprising: firstand second opposing capacitor electrodes, wherein the second capacitorelectrode is movable by a MEMS switch to vary spacing between the firstand second opposing capacitor electrodes the capacitor dielectricspacing, and thereby tune the capacitance; a third electrode facing themovable second electrode; and a tunable dielectric material and anon-tunable dielectric material in series between the second and thirdelectrodes, wherein the tunable dielectric material occupies a firstdimension of the spacing, and the non-tunable dielectric materialoccupies a second dimension of the spacing, the third electrode beingconfigured and arranged to electrically control the tunable dielectricmaterial, wherein the second capacitor electrode is configured andarranged to vary spacing in response to a first applied voltage for afirst continuous range of adjustment of the capacitance of the MEMScapacitor and wherein the tunable dielectric material is configured andarranged to be tuned, via the third electrode, in response to a secondapplied voltage, for a second continuous range of adjustment of thecapacitance of the MEMS capacitor, thereby to provide a continuousanalogue range of adjustment including the first and second ranges. 2.The capacitor as claimed in claim 1, wherein the non-tunable dielectricdimension is less than one third of the total effective actuationelectrode spacing when the movable electrode is at the positioncorresponding to maximum electrode spacing.
 3. The capacitor as claimedin claim 1, wherein the tunable dielectric material is a solid and thenon-tunable dielectric material is a gas.
 4. The capacitor as claimed inclaim 1, further comprising a dc voltage source connected to the firstand second capacitor electrodes and configured and arranged to controlthe MEMS switch to tune the capacitance between the first and secondcapacitor electrodes by moving the second capacitor electrode relativeto the first capacitor electrode.
 5. The capacitor as claimed in claim1, wherein the second capacitor electrode comprises a first ac electrodeportion facing the third electrode and a second dc electrode portionfacing the first electrode.
 6. The capacitor as claimed in claim 1,further including a plurality of spring portions spacedcircumferentially around the second capacitor electrode and configuredand arranged to suspend the second capacitor electrode.
 7. The capacitorof claim 1 as part of an apparatus comprising: an antenna device, afirst tunable circuit including a capacitor for a transmission channel,and a second tunable circuit including a capacitor for a receptionchannel, the capacitor for the transmission channel and the capacitorfor the reception channel both as recited in claim
 1. 8. The capacitorof claim 1, further comprising a tunable capacitor network including aplurality of tunable capacitors, wherein the plurality of capacitors areelectrically connected in parallel.
 9. The capacitor of claim 1, furthercomprising a tunable capacitor network including at least one staticcapacitor, wherein the at least one static capacitor is part of thetunable capacitor network.
 10. The capacitor as claimed in claim 1,further including circuitry configured and arranged to apply a first DCvoltage across the first and second electrodes to vary the spacing andprovide the first continuous range of adjustment, and to independentlyapply a second DC tuning voltage to the third electrode to set adielectric characteristic of the tunable dielectric material and providethe second continuous range of adjustment.
 11. The capacitor as claimedin claim 1, further comprising a first circuit connected to the firstand second capacitor electrodes and configured and arranged to vary thespacing by applying the first applied voltage across the first andsecond capacitor electrodes, and a second circuit connected to the thirdelectrode and configured and arranged to tune the tunable dielectricmaterial by applying the second applied voltage to the third electrode.12. The capacitor as claimed in claim 1, wherein the tunable dielectricmaterial comprises a ferroelectric material.
 13. The capacitor asclaimed in claim 12, wherein the tunable dielectric material comprisesBST.
 14. The capacitor as claimed in claim 1, wherein the first andthird electrodes are provided on a static substrate, and the secondelectrode comprises a sprung structure suspended over the staticsubstrate.
 15. The capacitor as claimed in claim 14, wherein only thethird electrode is covered by the tunable dielectric material.
 16. Thecapacitor as claimed in claim 14, wherein the first and third electrodesare covered by the tunable dielectric material.
 17. The capacitor asclaimed in claim 16, further comprising a fourth electrode provided overthe tunable dielectric only above the third electrode.
 18. The capacitoras claimed in claim 14, wherein the third electrode is covered by thetunable dielectric material and the first electrode is covered by adifferent dielectric material.
 19. The capacitor as claimed in claim 18,wherein the second dimension is g, which satisfies:g<(g2+gd2/∈_(d2))/3, in which g2 is a non dielectric gap between thesecond and third electrodes, gd2 is the dielectric thickness of thedielectric material covering the third electrode and ∈_(d2) is thepermittivity of the dielectric material covering the third electrode.20. The capacitor as claimed in claim 1, wherein the movable secondelectrode is shaped such that a different gap is provided between thefirst and second electrodes and between the second and third electrodes.21. The capacitor as claimed in claim 20, wherein the gap between thefirst and second electrodes is larger than the gap between the secondand third electrodes.
 22. A MEMS tunable capacitor comprising: first andsecond opposing capacitor electrodes, wherein the second capacitorelectrode is movable by a MEMS switch to vary spacing between the firstand second opposing capacitor electrodes, and thereby tune thecapacitance; a third electrode facing the movable second electrode; anda tunable dielectric material and a non-tunable dielectric material inseries between the second and third electrodes, wherein the tunabledielectric material occupies a first dimension of the spacing, and thenon-tunable dielectric material occupies a second dimension of thespacing, the third electrode being configured and arranged toelectrically control the tunable dielectric material, an ac rf voltagesource which includes a dc component and is connected to the thirdelectrode, the voltage source being configured and arranged to apply avoltage to set a dielectric constant of the tunable dielectric material;and wherein the second capacitor electrode is configured and arranged tovary the spacing in response to a first applied voltage for a firstcontinuous range of adjustment of the capacitance of the MEMS capacitorand wherein the tunable dielectric material is configured and arrangedto be tuned, via the third electrode, in response to a second appliedvoltage, for a second continuous range of adjustment of the capacitanceof the MEMS capacitor, thereby to provide a continuous analogue range ofadjustment including the first and second ranges.
 23. An apparatuscomprising: a capacitor circuit having first, second, and thirdelectrodes, the second electrode being separated from each of the firstand third electrodes by an electrode spacing; a tunable dielectricmaterial and a non-tunable dielectric material in series between thesecond and third electrodes, the tunable dielectric material occupying afirst dimension of the spacing, and the non-tunable dielectric materialoccupying a second dimension of the spacing; a first circuit connectedto the first and second electrodes and configured and arranged to applya voltage across the first and second electrodes, the second electrodebeing configured and arranged to move in response to the voltage; asecond circuit connected to the third electrode and configured andarranged to tune a dielectric constant of the tunable dielectricmaterial by applying a voltage to the third electrode; and the first andsecond circuits being configured and arranged to tune the capacitance ofthe capacitor by applying a voltage across the first and secondelectrodes to move the second electrode relative to the first and thirdelectrodes, and by applying a voltage to the third electrode to tune thedielectric constant of the tunable dielectric material.